1. Field of the Invention
The present invention relates to the field of printed circuit assembly construction. More specifically, the present invention relates to component and lead soldering techniques for preventing solder reflow during serially performed soldering steps.
2. Description of the Related Technology
In the manufacture of electrical and electronic products, it is essentially universal practice to create circuits by soldering components to printed circuit boards which incorporate conductive traces interconnecting the components in the desired manner. In some instances, components are individually hand soldered to their appropriate locations. In other applications, several components are placed on a printed circuit board at once, and are soldered in place essentially simultaneously by a wave soldering method, or by an infrared or convection oven reflow technique. Reflow methods are especially common in printed circuit assemblies which include surface mount components.
In a typical reflow procedure, the pads of the printed circuit board traces are coated with a solder paste by a stencil or screening process. Circuit components such as resistors, capacitors, and the like are then secured to their appropriate positions, typically with a small amount of adhesive. The assembly is then placed inside an oven, where the solder paste is raised to a temperature above its temperature of "liquidus", the temperature at which the solder is completely liquid. Upon cooling, the solder solidifies, securing the components to the pads.
In many modern electronic applications, a printed circuit assembly as described above is packaged or encapsulated to form a single electronic module having terminals and/or leads for external connection to other circuitry. Such a module can be then used as a component of another, "downstream", printed circuit assembly. In this case, the terminals or leads of the module are soldered to another printed circuit board at a downstream manufacturing facility. This soldering process may again comprise hand, wave, or reflow soldering techniques. It can be appreciated that during this subsequent soldering procedure performed on the module, it is desirable to have the solder internal to the module remain below its temperature of "solidus", the temperature at which the solder alloy is entirely solid. If the solder internal to the module is raised above the solidus temperature, migration and cracks will begin to form, reducing the stability of the internal solder connections. If the liquidus temperature is approached, the solder internal to the module may reflow again, forming internal short circuits and causing module failure.
Efforts to resolve this problem have focused on the use of a module solder system which have higher solidus and liquidus temperatures than the soldering system used to secure the module to the other printed circuit board. Most common solders use alloys of tin (Sn) and lead (Pb) having a solidus temperature of approximately 183 degrees C. A 63% tin, 37% lead (63 Sn/37 Pb) alloy is a common formulation, and ratios can often range from 40% to 60% Pb, with the balance being Sn. Although these tin-lead alloys are most common, other elements can be added or substituted for all or part of the Pb or Sn to produce solders with different mechanical strengths, grain sizes, or liquidus/solidus temperatures. Such elements include, for example, cadmium (Cd), bismuth (Bi), antimony (Sb), and silver (Ag).
Most downstream printed circuit assembly manufacturers use a Sn/Pb alloy solder at a reflow oven temperature of approximately 220 to 235 degrees C. To try to avoid module reflow under these oven conditions, a higher temperature module soldering system using an 85 Pb/10 Sb/5 Sn has been employed in some commercially available prior art modules. This solder has a solidus temperature of approximately 239 degrees C., and a liquidus temperature of approximately 243 degrees C. Even though modules using this solder alloy have been produced using pads plated first with nickel and then gold (thereby eliminating the thin coating of Sn/Pb solder typically placed on bare copper printed circuit board pads), these modules have been found to exhibit undesired solder reflow and its associated module failure in many downstream manufacturing environments which use standard Sn/Pb solder alloys.
Another prior art system, one that is currently produced by the assignee of the present application, uses 88 Pb/10 Sn/2 Ag solder on pads treated with a standard Sn/Pb solder dip rather than nickel and gold plate. The 88 Pb/10 Sn/2 Ag solder alloy has a liquidus temperature of approximately 299 degrees C. This solder does not have subsequent reflow problems in downstream manufacturing, but the initial reflow temperature of 320 to 330 degrees C. required with this solder renders the use of expensive high temperature printed circuit boards 30 such as epoxy/polyphenylene oxide resin boards, available, for example, as type GETEK (TM) from General Electric. This high initial reflow temperature also tends to over-stress the components mounted on the printed circuit board 30, as they are commonly rated by component manufacturers to withstand only approximately 260 degrees C. during soldering.
There is accordingly a need in the art for an improved module soldering system which does not have reflow problems during downstream incorporation into another printed circuit assembly which utilizes standard Pb/Sn soldering methods. The module soldering technique should also use a reflow temperature which can be tolerated by standard inexpensive printed circuit boards and circuit components.